1. Field of the Invention
The present invention relates to a dynamic pressure-type hydrodynamic bearing device, as well as a spindle motor and information device using the same.
2. Description of the Prior Art
A hydrodynamic bearing device comprises a shaft and a sleeve that supports the shaft, and a lubricant that is interposed in the gap between the two parts. With rotation of the shaft, the lubricant is gathered up by dynamic pressure-generating grooves that are formed on the shaft or sleeve, and generates pressure such that the shaft is supported within the sleeve without coming into contact therewith. As a result, when high-speed rotation is attained, ambient noise during the rotation can be alleviated.
A spindle motor equipped with such a hydrodynamic bearing device can provide the requisite rotational accuracy with an increased recording density of the medium, and can furthermore provide excellent shock resistance and quietness. Thus, it can be used in a majority of motors for application in such representative magnetic disk devices as information technology equipment and audio-visual equipment.
However, in the particular case of a spindle motor equipped with this type of hydrodynamic bearing device used in magnetic disk devices, an electrostatic charge is generated by the flow of the lubricant that is interposed between the shaft and the sleeve and the air friction of the magnetic disk due to the high-speed rotation without contact between the sleeve and the shaft of the hydrodynamic bearing device, and this electric charge will be accumulated within the device. This accumulated electric charge can be discharged between the magnetic disk connected either to the shaft or the sleeve and the record/replay head, raising a concern about record/replay failures or damage to the record/replay head.
Countermeasures against this that have been proposed include magnetic disks where organic polymers as conductivity-enhancing additives are added to the lubricant (for example, see PCT 2000-500898 Official Bulletin citation), hydrodynamic bearing spindle motors where antistatic additives are added to the lubricant (for example, see Japanese published unexamined application No. 2001-208069), hydrodynamic bearing devices having lubricants that are formulated with specific additives (for example, see Japanese published unexamined application No. 2003-171685), and the like. In this type of technology, various additives are added to the lubricant in order to increase the conductivity or antistatic effect so that either a ground is provided for the electric charge within the device or the electrostatic charge is suppressed.
However, the viscosity of the lubricant will rise after adding more than a certain amount of a high molecular weight or a high viscosity compound as an additive, which leads to the problem of increased torque in the bearing device (in other words, increasing the power consumption).
Moreover, with the objective of increasing the specific effects of the additive (conductivity from a conductivity-enhancing additive, or prevention of static charge from an antistatic additive), the heat resistance and durability of the additive itself can exert an influence on the other functions of the lubricant. Furthermore, as the additive undergoes degradation, the reduced effectiveness of the additive can cause a progressive degradation of the entire lubricant, leading to the problem that long-term reliability cannot be achieved for the device.
In addition, in recent years, the demand has grown for magnetic disk devices that are increasingly miniaturized, more energy-conserving and progressed with operational lifetime, for decreased power consumption of motor and improvement of endurance for the spindle motor that is the main component.
For this reason, esters such as dioctyl sebacate (DOS), dioctyl azelate (DOZ), and dioctyl adipate (DOA) have been proposed for use as lubricants in hydrodynamic bearing devices. Moreover, esters obtained from neopentyl glycol and C6 to C12 monovalent fatty acids and/or their derivatives for use as lubricants in hydrodynamic bearing devices (see for example Japanese published unexamined application No. 2001-316687), the use of esters represented by the generic formula R1—COO-(AO)n—R2 as lubricants for bearings (see for example Japanese published unexamined application No. 2002-206094) have been proposed. The use of low viscosity lubricants can also result in reduced torque (in other words, low power consumption).
However, while it is possible to reduce the torque in such conventional hydrodynamic bearing devices, since the heat resistance of the lubricant is low (vapor pressure is high), the amount of evaporation will be significant when used over a long period, and it will not be possible to maintain the quantity of lubricant required for stabilized rotation of the bearing device. Consequently, there will be problems with the device having inadequate reliability and the operational lifetime will be shorter. As a countermeasure to the amount of evaporation, one can consider a method by which the above requirement is addressed by adding an excess of the lubricant. However, this approach will entail problems in that this additional amount can increase the torque and bring a higher cost, and accommodating the additional space will make miniaturization more difficult.